1. Field of the Invention
The present invention relates to a helical scan type magnetic tape reproduction apparatus and a helical scan type magnetic tape reproduction method suitable for use, for example, for reproduction of a magnetic tape on which data of a computer or the like are recorded.
2. Description of the Related Art
In the past, a helical scan type magnetic tape recording and reproduction apparatus is used as an apparatus for recording and storing data of a computer. A magnetic tape recording and reproduction apparatus of the type mentioned has such a configuration as described below.
First, recording of data is described. A recording head and associated elements are shown in FIG. 11. Referring to FIG. 11, a rotary drum 2 is mounted for rotation on a fixed drum 1, and a recording head 3 is mounted on the outer periphery of a lower portion of the rotary drum 2. Actually, while a pair of such recording heads 3 is provided, the other one of the recording heads 3 is provided on the opposite side displaced by 180 degrees. Accordingly, as shown in FIG. 12, data recording on a magnetic tape 4 is performed by rotating the rotary drum 2 at a fixed speed in a state wherein the magnetic tape 4 is wrapped on the rotary drum 2 and the fixed drum 1 while the magnetic tape 4 is in a traveling state at a fixed speed. A pair of guide posts 6 and 7 guide the magnetic tape 4.
On the other hand, data reproduction (including read after write when data recording is performed) is performed, for example, by a pair of reproduction heads provided at positions displaced by 180 degrees from each other on the rotary drum 2. Upon the data reproduction, the reproduction heads are controlled so as to pass just above the central position of a track 8 as shown in FIG. 13. A state wherein the reproduction heads as just described assumes such a moving locus as just described is called on-track. On the other hand, a state wherein the moving locus of the reproduction heads is offset from a track as shown in FIG. 14 is called off-track.
In FIGS. 15A and 15B, reproduction signal envelopes in the on-track state and the off-track state are illustrated. As recognized from FIGS. 15A and 15B, the track is not traced over the full width thereof by the reproduction heads in the off-track state. Therefore, the amplitude level of a reproduction signal drops.
Thus, in order to avoid the off-track, tracking servo (magnetic tape feeding phase control) is applied to feed a magnetic tape. A servo system for such tracking servo is shown in FIG. 16. Referring to FIG. 16, fixed rotary servo is applied to a drum motor 9, and phase servo is applied to a capstan motor 10 for feeding the magnetic tape 4 together with a pinch roller 11. A particular example of capstan phase servo control is illustrated in FIG. 17.
While, as shown in FIG. 17, the moving locus of the reproduction heads is indicated by an arrow mark of a solid line, it is recognized from FIG. 17 that a track phase b is in the on-track state while track phases a and c are in the off-track state. Accordingly, in order to achieve a state wherein the track phases a and c usually and individually coincide with the track phase b, the feeding amount of the magnetic tape 4 may be adjusted so that the phase relationship between the reproduction heads and the magnetic tape 4 may be such as the track phase b. In other words, the phase servo is applied to the capstan motor 10. Such a capstan motor phase servo as just described is called tracking servo.
Phase detection is demanded in order to apply the phase servo. The principle of TATF (Timing Auto Track Finding) which is one of phase detection methods is illustrated in FIGS. 18A and 18B. Referring to FIG. 18A, as well as reproduction heads 21 and 22, a PG magnet 24 for detecting the phase of the rotary drum 2 is mounted on the rotary drum 2, and a PG pulse is generated from a PG sensor 23 every time the PG magnet 24 reaches a specific rotation phase. Referring to FIG. 18B, a marker (signal) 28 is recorded in advance at the same position of the tracks 8 on the magnetic tape 4. The reproduction signal from the reproduction head 21 is processed by a marker detection circuit 26 through a rotary transformer 25 and a reproduction circuit 29 so that a marker signal (marker signal pulse) can be detected. Consequently, a time length t from a point of time of generation of the PG pulse to detection of the marker signal can be measured by a time measurement circuit 27.
It is recognized that, although the marker 28 is recorded at the positions same as each other on the tracks 8 as shown in FIG. 18B, the time length t is given in a relationship of ta>tb>tc among the tracks a, b and c. In other words, if tb is obtained as the time length, then it can be decided that the reproduction head is in the on-track state, but, if ta is obtained as the time length, then it can be decided that the reproduction head is in the off-track state and the magnetic tape 4 is in a leading state with respect to the reproduction head 21. On the other hand, if tc is obtained as the time length, then it can be decided that the reproduction head is in the off-track state and the magnetic tape 4 is in a delaying state with respect to the reproduction head 21. The phase relationship between the reproduction head 21 and the track 8 can be measured by such a principle as described above, and, if it is recognized in advance that tb indicates the on-track state, then the tracking servo should be applied so as to implement t=tb.
Here, where it is tried to particularly calculate the values of the time lengths ta to tc with reference to FIGS. 19A to 19C, the following conditions are provided by the AIT3 format:
Track leading angle=6 degrees
Track width=5.5 μm
θm=θs=30 degrees
Rotary drum diameter=40 mm
Rotary drum speed=6000 rpm=100 rps
Marker position: position of 10 degrees from track top position
Off-track of tracks a and c: ±1 μm
Accordingly, since the time length tb is calculated as a period of time necessary for rotation by 10 degrees from the generation point of the PG pulse, the time length tb is obtained as tb=0.27777777 ms (=(1/100 rps)×(10 degrees/360 degrees)). Further, the angle conversion value of 1 μm off-track is obtained as 0.02727 degrees (=1 μm/Tan(6 degrees)/(π×40 mm)×360 degrees), and the time conversion value of the angle conversion value is obtained as 758 ns (=0.02727 degrees/360 degrees×(1/100 rps)). Therefore, the times lengths are obtained as ta=0.2785358 . . . ms, tb=0.2777777 . . . ms, and tc=0.02770198 . . . ms.
If it is assumed that the marker 28 is recorded at the positions of the tracks 8 same as each other as described above, the TATF operation according to the existing technique is such as described below. In the TATF operation, TATF learning is performed first, and then the TATF operation is performed. In the TATF learning, the capstan motor is placed in a free rotation (non-tracking) state. Accordingly, as indicated by a reproduction head traveling locus (indicated by an arrow mark of a solid line) in FIG. 20, the tracks 8 are scanned by the reproduction head in various track phase states to measure the time length t and the error rate. However, a marker is not necessarily detected every time the tracks 8 are scanned by the reproduction head. This is because the tracks 8 are not necessarily scanned usually in a state near to the on-track state or the off-track state.
It is to be noted that techniques regarding the TATF are disclosed individually in Japanese Patent Laid-Open No. Hei 6-96500 and Japanese Patent Laid-Open No. Hei 7-29256 (hereinafter referred to as Patent Documents 1 and 2, respectively). Further, techniques for moving a head in the track widthwise direction are disclosed individually in Japanese Patent Laid-Open No. Hei 11-259835 and No. Hei 4-78016 (Japanese Patent No. 2589859) (hereinafter referred to as Patent Documents 3 and 4, respectively). Further, a pilot signal is used as tracking information for DT (Dynamic Tracking) servo as disclosed in Japanese Patent Laid-Open No. Hei 6-349156 (Japanese Patent No. 3036298) (hereinafter referred to as Patent Document 5).